Method for measurement of relative humidity, in particular in radiosondes

Abstract: Method, in particular in radiosondes, for measurement of relative humidity by using a capacitive humidity detector (10) or a combination of detectors (10₁,10₂;10P), as well as humidity detectors. In the detector, between the capacitor plates, an insulating material (12) is used, whose permittivity is a function of the amount of water absorbed by the insulating material (12). The detector (10) or the combination of detectors is heated in order to remove any ice, frost or condensed humidity deposited on the face and/or in the environment of the detector. The detector capacitance (C M ) or a part (C₁...C N ) of same is heated, in time and/or in place, periodically by means of electric current (I). The detecting of the detector capacitance (C M ) is carried out without measurement of the detector temperature after the detector capacitance or a part (C₁...C N ) of same has cooled down substantially to the temperature of the environment.

Description: The invention concerns a method of measuring relative humidity. In the prior art, a number of different electrically detected temperature and humidity detectors are known whose impedance changes as a function of the quantity to be measured. Such humidity detectors are known, e.g. from the US Patents Nos. 3,168,829 and 3,350,941 and from the applicant's Finnish Patent No. 48, 229. As is known in the prior art, for measurement of temperature, capacitive detectors are used, which are usually based on the fact that the permittivity of the insulating material between the capacitor plates is dependent on the temperature, in which case the capacitance detected from the terminals of the detector also depends on the temperature. The FI Patent No. 48,229 is related to the prior art concerned in the present invention, in which patent a capacitive humidity detector is described in which the dielectric insulating material is a polymer film, whose permittivity is a function of the amount of water absorbed by the polymer film. In the detectors described above and also in other detectors based on change in impedance, undesirable phenomena occur, which include freezing and wetting of detectors, radiation error, slowness of the detectors, and hysteresis. In the applicant's FI Patent No. 58,402, a method is described for reduction of undesirable properties produced by reversible changes in an electric humidity detector based on change in impedance, in particular in a capacitive humidity detector, in which the material sensitive to humidity is an organic polymer, which is heated, at least with higher humidities, to a temperature higher than the temperature in the environment of the humidity detector. If necessary, the heating power of the detector can be regulated as a function of the humidity to be measured. In said FI patent, the temperature of the humidity detector and/or the outside temperature is/are measured, and this or these auxiliary quantities are utilized in the computing of the humidity measurement values. In said FI patent it is stated that the heating of the detector can be switched off for the time of the measurement, but it is not stated more specifically in what way and for what purpose this switching-off takes place. In any case, in said FI Patent 58,402, measurement of the temperature of the detector is always required, and the measurement result is corrected on the basis of said measurement of temperature because, owing to its raised temperature, the detector "shows" an excessively low value of relative humidity. Regarding the prior art, reference is made further to the applicant's FI Patent No. 58,403 (corresponding GB Patent No. 2,047,431), wherein a regulating device in a humidity detector is described, which comprises a bridge connection or equivalent, which includes temperature-dependent resistor elements, by whose means the temperature outside the detector and the temperature T s of the detector itself are detected, the differential voltage in said bridge connection being used as a feedback signal, by whose means the electric power that heats the detector is regulated. As the heating resistor of the detector, such a resistor element of positive temperature coefficient is used, for example a platinum resistor, as operates at the same time as the sensor of the temperature T s of the detector. As the sensor of the outside temperature T a , such a resistor-thermistor assembly is used, which is placed in an opposite position in relation to the location of the last-mentioned resistor element at the other branch of the bridge connection, that a certain function T s = f(T a ) characteristic of each detector type comes true by means of the regulator device. With respect to the prior art related to the present invention, reference is made further to the applicant's FI Patent No. 85, 770 (corresponding US Pat. 5,156,045), wherein a method is described in connection with impedance detectors for radiosondes, in which method the temperature of the detector or detectors is measured by means of a thermocouple, in which the connection of one branch of its thermoelements is placed in connection with, or at the proximity of, the detector to be measured, and in which thermocouple the connection of the other branch is placed in the atmosphere surrounding the detector, and in which method, by means of said thermocouple, the difference between the temperature prevailing in connection with the detector and the temperature of the surrounding atmosphere is observed, an electric signal that represents said difference acting upon the output signal of the measurement coupling of the radiosonde, which signal includes the information on the meteorological quantity or quantities to be measured by means of the detector or detectors. With respect to the prior art, reference is made further to the EP-patent publication 356682, wherein a resistive detector which is heated by an electric current conducted through a resistor in the detector is described. A heating period, a cooling period and a humidity measurement period is shown. The heating period is a fraction of the whole measurement cycle. The moisture sensitive element in the detector is heated to a temperature over the temperature of the environment in order to shorten the time during which the moisture sensitive element gives wrong humidity values when there is a change in the environment from higher humidity to lower humidity (desorption). A measurement of the temperature based on the resistivity of the resistor may also be included, which makes it possible to determine the optimal time to start the humidity measurement or to make a temperature compensation to the measured humidity value. With respect to the prior art, reference is made further to the US-patent 5,296,819 claiming priority from JP 3-179006 filed on June 25, 1991 and published as JP 5099877. The US-patent shows a capacitive humidity detector having a moisture sensitive material, the permittivity of which is a function of the amount of water absorbed by said material between the capacitor plates. The moisture sensitive film in the detector can be heated by conducting electric current through a resistor in order to restore the initial capacitance/moisture-ratio of the moisture sensitive film, which has drifted due to exposure to a high temperature and a high humidity, an organic solvent atmosphere, cigarette smoke or gas, or the like. The heating and cooling operations can be performed at optimal temperatures when a temperature measuring circuit is also used in the detector. In a situation where a capacitive humidity detector, for example in a radiosonde flying in a supercooled claud, is subjected to a high humidity and frost or ice is gathered and condensed on the active face of the detector or on the structures in its environment the detector operation deteriorates. When such a situation of disturbance is over, it takes a long time before the frost or ice has evaporated, during which period of time the detector, of course, gives an erroneous message, indicating an excessively high humidity. Some of the drawbacks mentioned above can be avoided by means of the heating of the capacitive humidity detector described in the above FI Patent 58, 402, but it still remains a problem with no satisfactory solution that, in order that a sufficiently accurate measurement of humidity could be provided, the temperature of the humidity detector must also be known highly accurately. In order that an accuracy of ∼ 1...2 % of measurement of relative humidity could be achieved, it must be possible to measure the temperature of the detector at a precision of ∼ 0.1°C. In the measurement of the temperature, there may be a higher absolute error, but the difference in temperature as compared with the environment must be known at said precision. The object of the invention is to provide a method of measurement by whose means relative humidity can be measured at least at the precision of ~ 1...2 % mentioned above without measurement of the detector temperature. It is a further object of the invention to provide a method of measurement that is particularly well suitable for disposable radiosondes. In view of achieving the objectives stated above and those that will come out later, the method of the invention is defined in claim 1. When the method in accordance with the invention is applied, the stage of measurement of the detector temperature and the equipment and computing arrangements necessary for said stage can be omitted completely, as a result of which the steps of carrying out the method and the detector constructions and the related other systems are simplified substantially and, yet, the above accuracy of measurement is achieved, which is, as a rule, adequate in sonde operation. In the invention, as heating energy, expressly electric current is used, preferably direct current, which is passed, in accordance with the method of the invention, into a resistive heating resistor or into different combinations of heating resistors placed in connection with the detector or combination of detectors. The heating of the humidity detector, applied in the invention, is needed in order to remove any frost or ice, which is detrimental in view of the measurement of humidity, from the detector. A detector assembly used in the method consists of a part that measures the humidity and of a heating part. These may be separate constructions, or some construction of the humidity measurement may also be used for heating. If necessary, the heating capacity may also be regulated in various ways. In the realization of the heating of a humidity detector to be used in the method in accordance with the invention, measurement and regulation electronics may be used as an addition to an ordinary sonde application. When needed, the necessity of heating is detected, and the heating elements itself may operate as the detector of the heating requirement so that frost or ice produces a change in the heating rate of the element, and the slowing down of the specific resistance of the resistor piece in the element is measured by means of the electronics. The electronic system allows the heating to be continued or switches it off, as necessary. The heating cycle can be carried out once during one measurement cycle of the sonde, or otherwise. The heating capacity can be regulated when necessary. If necessary, the magnitude of the capacity can be measured, for example, by means of a resistor that is heated simultaneously with the heating. This information can also be used as a measure of freezing efficiency of supersaturation. In the method in invention, the advantage is also obtained that various electrical phenomena related to the electric current used for the heating do not interfere with the measurement of the detector capacitance, which is also important in the respect that said detector capacitances used in the measurements of relative humidity are quite low, their capacitance value being, as a rule, in the range of 1...100 pf. In the following, the method in the invention will be described in detail with reference to some preferred exemplifying arrangements illustrated in the figures in the accompanying drawing The specific embodiments of humidity detectors and measurement systems described with reference to Figs. 1-6 and 10 do not as such fall within the scope of the claims. Fig. 1A is a top view of a first embodiment of a detector used in the method of the invention. Fig. 1B shows the same as Fig. 1A as a side view. Fig. 1C shows an equivalent circuit of the detector as shown in Figs. 1A and 1B. Fig. 2A shows a second embodiment of a detector used in the method of the invention. Fig. 2B shows the same as Fig. 2A as a side view. Fig. 2C shows an equivalent circuit of the detector as shown in Figs. 2A and 2B. Fig. 3A is a top view of a third embodiment of a detector used in the method of the invention. Fig. 3B shows the same as Fig. 3A as a side view. Fig. 4A is a side view of a humidity detector for the method in accordance with the invention in which the heating resistor is wound around the detector. Fig. 4B shows the same as Fig. 4A as a top view. Fig. 5A is an illustration similar to Fig. 4A of a modification of the detector shown in Fig. 4A. Fig. 5B is a top view of a detector as shown in Fig. 5A. Fig. 6 is a block diagram illustration of a measurement system in accordance with the invention in which a humidity detector in accordance with a second embodiment is used. Fig. 7 shows the measurement signal of the humidity measurement of the system as shown in Fig. 6 as a function of time. Fig. 8 illustrates the measurement sequences of the system as shown if Fig. 6 on the horizontal time axis of Fig. 7. Fig. 9 shows the heating that is used in the measurement system as shown in Figs. 6, 7 and 8 and that is pulsated in accordance with the invention on the horizontal time axis of Figs. 7 and 8. Fig. 10 shows an exemplifying embodiment of an electronics wiring by whose means it can be detected if there is frost or ice on a detector as shown, for example, in Figs. 4A, 4B or 5A, 5B. The humidity detector shown in Fig. 1A and 1B is constructed on a stable glass substrate 10, onto one of whose plane faces the electrode 13a of the detector capacitance is applied, onto which electrode a plastic film 12 is fitted whose permittivity is a function of the amount of water absorbed by the film 12. Onto the film 12, a surface electrode film 14 is applied, which is penetrable by water but, yet, electrically conductive and which is in galvanic connection with a surface contact 13b. The capacitance C M to be measured is detected between the terminals c 1 and c 2 connected to the contacts 13a and 13b. The detector 10 as shown in Figs. 1A and 1B is provided with an electrical heating resistor 15, whose conductor pattern is applied onto the same face of the substrate 11 as the bottom electrode 13a. To the heating resistor 15, the electric heating current I, preferably direct current, can be passed through the terminals r 1 and r 2 connected to the contact patterns 15a and 15b. The electric equivalent circuit of the humidity detector 10 as shown in Figs. 1A and 1B is illustrated in Fig. 1C. The heating current I passed to the heating resistor R produces a heating power W = 1 2 x R, which heats the active film 12 of the capacitance to be measured, melts any ice or frost that may be present on it and evaporates any extra moisture from the face of the film 12. Figs. 2A and 2B show a second humidity detector 10 for the method in accordance with the invention, in which no separate heating resistor is used, but the heating resistor R consists of an electrically conductive surface electrode 14. The opposite edges if the surface electrode 14 are connected to conductor patterns 13b and 15a, the heating current I being fed through the terminals r 1 and r 2 connected to said conductor patterns, which current I, when it flows through the conductive resistive material of the surface electrode 14, produces the desired heating effect W = I 2 x R, wherein R = the resistance measured from the terminals 13b, 15a of the surface electrode. The surface electrode 14 is a very thin, "perforated" film penetrable by moisture and made, e.g. of gold, which film is, however electrically continuous and, also electrically conductive. The electric equivalent circuit of the humidity detector 10 as shown in Figs. 2A and 2B is shown in Fig. 2C, from which it is seen that the capacitance C M to be measured and the heating resistor R that consists of the surface electrode have a common terminal c 2 = r 2 . Figs. 3A and 3B show a detector 10 that is in the other respects similar to Figs. 1A and 1B except that the heating resistor 15 is placed on the opposite face of the glass substrate 11 in relation to the electrodes 13a, 14 and to the active film 12. The equivalent circuit of the humidity detector as shown in Fig. 3A and 3B is similar to that shown in. Fig. 1C. In view of achieving the objectives of the invention, the measures 1 x m x s of the detectors shown in Figs. 1, 2 and 3 are relatively small, so that both the size and the thermal mass of the detector can be made small and, thus, such that it is heated and cooled sufficiently rapidly. Typically, the dimensions 1 x m x s given above are ≈ 4 mm x 1.5 mm x 0,2 mm. In Figs. 4A and 4B, a detector 10 for the method in accordance with the invention is shown that is attached to support strips 16a and 16b, which are made of an insulating material. Around the support strips 16a, 16b and, at the same time, around the active detector 10, a resistor wire 15 is wound, which forms the resistor R that heats the detector, the heating current I being passed to the terminals r 1 and r 2 of said resistor R. Figs. 5A and 5B show a heated detector construction which is in the other respects similar to that shown in Figs. 4A and 4B except that the active detector 10 is attached to a unified strip 16, around which, like around the detector 10, a resistor wire 15 is wound, which forms the heating resistor R of the detector 10. The electric equivalent circuit of the detectors as shown in Figs. 4 and 5 is similar to that shown in Fig. 1C. Thus, according to Figs. 4 and 5, the detector 10 proper, the heating resistor 15, and its possible support constructions 16a, 16b are, differing from Figs. 1 to 3, constructions separate from one another and manufactured in different stages of manufacture. It is an advantage of the constructions as shown in Figs. 4 and 5 that, as the humidity detector 10, it is possible to use detectors similar to the prior art, for example the detectors marketed by the applicant under the trade mark "Humicup". However, a particular advantage of the constructions as shown in Figs. 1 to 3 is their small size and low thermal mass, which are of substantial advantage in a number of different applications of the invention, as will come out in more detail later. In the arrangement shown in Fig. 6 one humidity detector u = 10 is used, which is connected with a heating resistor 15, which is heated from the heating-power unit 21 by means of the heating current I, controlled by the unit 22. According to Fig. 6, the system comprises a measurement-sequence unit 43, which controls a multiplexer 41 and a control unit 22 for the heating of the detector 10, to which control unit the heating power is fed from the unit 21. In connection with the humidity detector 10, there is a heating resistor 15, into which a heating current I is fed being controlled by the unit 21. The multiplexer 41 connects all the different detectors of the sonde to the measurement and to the telemeter connection alternatingly, said detectors being denoted in Fig. 6 with S, P, K 2 , T, U = 10, and K 1 . The detector S represents, e.g., the temperature detector of a pressure detector, the detector P a pressure detector, the detector T a temperature detector, and the detectors K 1 and K 2 represent precisely known reference and calibration detectors. Through the multiplexer 41, said detectors, which are usually capacitive detectors, are connected to a capacitance part which determines the frequency of the RC oscillator 42, whereby the frequency f M is obtained from the oscillator 42, which frequency is proportional to the quantity to be measured. The radio transmitter 36 is modulated with the frequency f M . Said units are placed in the radiosonde rising on support of a gas balloon, and in radio contact through a wireless telemeter link with the radio receivers 37 and the data processing units 38 at the ground station. By means of the unit 38, the physical quantities of the atmosphere detected by means of separate detectors are processed, indicated and displayed. Figs. 7, 8 and 9 are more detailed illustrations of the operation of the system as shown in Fig. 6. Fig. 7 is a graphic illustration of a typical measurement signal RH R received from the detector 10, which signal varies in sawtooth-wave form. At the time t 1 , the heating power W 0 = I 2 x R of the detector is switched on, and it remains on during the period of time t 0 , i.e. until the point of time t 2 , at which time the measurement signal RH R received from the detector is at the minimum. The heating power W 0 is typically in the range of W 0 ≈ 1...100 W, preferably in the range of W 0 ≈ 2...10 W. When the heating is switched off, the active film 12 in the detector 10 starts cooling, and then the reading of the detectors 10 increases exponentially and approaches the correct value RH M of relative humidity. The "reading" of the detector is carried out during the time period t 3 ...t 4 (stage u in Fig. 8), by which time the detector 10 has had time to cool and to be stabilized, because of its little thermal mass, sufficiently close to the temperature of the environment, in which situation the detector 10 provides a correct reading = RH M of relative humidity without measurement and compensation of temperature. The measurement stage t 3 ...t 4 is followed by a new stage of heating of the detector 10, and the steps stated above are repeated in the cycles T 0 in the way shown in Figs. 7, 8 and 9. As is seen best from Figs. 7 and 9, the duration t 0 of the heating stage is just a fraction of the duration T 0 of the measurement cycle T 0 , because of which the stabilization and cooling of the measurement detector 10, as shown in Fig. 7, is achieved between the heating cycles. The duration of a measurement cycle is in radiosondes typically of an order of T 0 = 0.2...2 seconds. When one detector is used, the duration of a heating cycle t 0 is typically of an order of t 0 ≈ 1 ms ... 100 ms. The ratio T 0 /t 0 is, as a rule, chosen in the range of T 0 /t 0 ≈ 10...3000, preferably in the range of T 0 /t 0 ≈ 100...250. In Figs. 6-9 a detector 10 of a size as small as possible is used, whose size is preferably similar to that illustrated in Figs. 1 or 2 and whose thermal mass is little, and which is additionally of low weight and of low cost of manufacture and, thus, particularly suitable for disposable sondes. The invention can be combined with a feature by whose means freezing or formation of frost on or in the vicinity of the active face of the detector can be detected. This feature can be accomplished, e.g., by means of a wiring similar to that shown in fig. 10, whose operation is characterized by measurement of the rate of heating of equivalent of the heating resistor, for example of the heating resistor 15 present in the constructions as shown in Figs. 4 and 5. The wiring shown in Fig. 10 comprises a feed unit 21 for heating power and a unit 23 for detecting the requirement of heating. By means of the unit 23, based on the rate of change in the resistance of the heating resistor 15, the requirement of heating is detected on the basis of the input signal U c of the unit 23. The unit 23 gives the heating-power regulation unit 22 and/or the unit 24 for regulation of the period of the heating pulse (optional) regulation signals c 1 and c 2 . The operation of the unit 23 is based on the fact that, if there is frost or ice in connection with the heating resistor 15 and in connection with the detector capacitance C M connected with said resistor 15, after switching-on of the heating pulse, the rate of increase of the resistance of the heating resistor 15 is substantially slower than in a situation in which there is no frost or ice on or near the heating resistor 15 and the active face of the detector capacitance C M , because said frost and ice contribute to the formation of a thermal mass that retards the heating of the detector resistor 15. At the beginning of the heating stage, when it is noticed that the rate of increase of the detector resistance 15, which can be indicated, e.g. with an invariable current I, as a rate of increase in the voltage U c , or in an equivalent way, is higher than a certain threshold value, the heating is discontinued under control by the signals c 1 and/or of the unit 23. On the other hand, if the rate of heating of the resistor 15 and the rate of increase in its resistance are lower than said threshold value, the periodic heating of the detector resistance 15 in accordance with the invention is continued under control by the regulation signals c 1 and/or c 2 of the unit 23 until said threshold value is surpassed and/or while following a preset sequence.

Claims: Method of measuring relative humidity by using a capacitive humidity detector (10), in conditions where ice or frost deposits on the surface of the capacitive humidity detector in particular in radiosondes, in which detector, between the capacitor plates a material (12) is used, the permittivity of which is a function of the amount of water absorbed by said material (12), and in which method the capacitance (C M ) of said detector (10) is heated, within a heating cycle (t 0 ), periodically by conducting an electric current (I) through a heating resistor (R) in order to remove any ice or frost deposited on the face or in the environment of the detector, in which method detecting of the detector capacitance (C M ) is carried out within a measurement cycle T 0 and without measurement of the detector temperature, and in which method the heating cycle (t 0 ) of the detector is a fraction of the measurement cycle T 0 of the detector, and the detecting (t 3 ... t 4 ) of the detector capacitance (C M ) is carried out after the heating cycle (t 0 ) in the final stage of the measurement cycle T 0 after the detector (10) has had time to be stabilized after said heating cycle (t 0 ) and to cool down substantially to the temperature of its environment, at which time the detector shows a correct humidity reading (RH M ), the duration T 0 of the measurement cycle being selected from the range of T 0 ≈0.2 ... 2 s, the duration t 0 of the heating cycle being selected from the range of t 0 ≈ 1 ... 100 ms and the ratio of the duration of the measurement cycle T 0 to the duration of the heating cycle t 0 being selected from the range of T 0 /t 0 =10 ... 3000. Method as claimed in claim 1, wherein said ratio T 0 /t 0 is selected from the range of T 0 /t 0 ≈100 ... 250.

Inventors: ANTIKAINEN VEIJO, JAUHIAINEN HANNU, PAUKKUNEN ARI, PONKALA JORMA, STORMBOM LARS, ANTIKAINEN, VEIJO, JAUHIAINEN, HANNU, PAUKKUNEN, ARI, PONKALA, JORMA, STORMBOM, LARS
Applicants: VAISALA OY
Application year: 1994
Main classification: G01N27
IPC codes: G01N27, G01W1